Can I charge LiPos from a 12v battery charger?

[owend]

Sorry, poor title but I don't know how to phrase it more clearly!

I'm new to the Dark Art of Electric Helis (started on a Blade mCX2, and now started emigrating to a Blade 450). I'm still rather confused about the battery side, my education sort of topped out at putting AAs in a Walkman so LiPos are completely new.

The battery the 450 came with is a 3S 11.1v 2200mAh 30C EFlite number and I got a Hacker 2400mAh 20C to double up with. I have the EFlite balanced charger, which needs a 12v 1.8A input, it says, and includes a croc-clipped connector; I have a small 12v lead-acid battery (4Ah) which charges one LiPo but sometimes needs a recharge itself halfway through!

My question is: when it's charging, the lead-acid battery shows 12.8v across the terminals. Can I connect the EFlite charger to the lead-acid's terminals while it's charging? My memory of school physics is that it should be OK, but I don't want to blow everything up without checking with you experts first.

Thanks in advance,

Owen

[Delarado]

You may find when you try to charge off a battery that is charging you might draw more current than the battery charger can provide and blow the fuse in it.

[cjcj1949]

It depends on the chargers. If you can set the car battery charger to charge at less amps than the lipo charger needs then that will be ok. If you have a good car battery charger it should charge to 14.4 V. Some hobby chargers than can charge lead acid will charge to 14V, some to 15V and some to 14.4V. Each lipo charger will work over a range of volts some up to 15V some to 18V or 24V. Some car battery chargers will keep charging and the volts could get very high. I'm not sure how you got your 12.8V, perhaps that was at the beginning of charge. See what it goes up to at end of charge.

[owend]

Thanks for the replies. cjcj1949: the charger is a mid-market one from Halfords some years ago (instruction leaflet lost); it's got switches for car or bike settings (on bike now), and lead-acid or gel (it's on lead-acid now), but doesn't mention 14.4v. The 12.8v was indeed at the start of charging; it's gone up to 13.96v after an hour of charge. The EFlite balanced charger requires 12v 1.8A according to the Blade instruction book, but the charger itself is marked "11.5v - 15v input"; the connector to go into the balanced charger reads 13.96v too (no great surprise, it's a straight wiring job to the croc clips!).

Delarado: is current likely to be an issue? I don't know how much a LiPo would draw from the charger, the Halfords' charger has a 20A fuse fitted and is running off a 13A socket with no problems.

You can see I'm learning! The good news is that I ran the 450 for 12 mins (two batteries), no crashes, and flying round the lawn under reasonable control, at up to a metre altitude - its second day out!

Owen

[owend]

After the advice here, I tried it: "big" charger connected to battery terminals, "small" charger's croc clips on battery terminals. It works fine, the voltage across the battery terminals doesn't go over 13.6v at any time. Without the "small" charger, ie just charging the battery on the bench, the voltage goes up to 14.4v when it's fully charged, which is still below the voltage printed on the "small", EFlite, charger, even if the instruction book quotes 12v.

No blown fuses or LiPos!

Owen

[cjcj1949]

If the charger doesn't go above 14.4V your ok. It's a proper car battery charger and it'll be cutting it's charge current down to zero at 14.4V. Keep a check on it though and there's no point leaving it charging if you don't have to. (So long as the car battery is as charged as you want. It needs to be charging at less than 500ma at 14.4V to guarantee a reasonable degree of charge.

[owend]

Yes, I'm keeping a close eye on the LiPos while they're charging, after reading all the horror stories of exploding batteries, so everything is switched off after the LiPos are charged and balanced.

[cjcj1949]

I meant the volts on the 12V car battery. Leave it on charge without the charger connected and make sure it stays below 15V. You may have an old time switch around and you can set that to give just as much as you need. You will need a bigger battery at some point and you may be able to get some 17Ah types that are due to be dumped after being used in disabled vehicles. They will still give about 10AH at 5amps and they might give them to you if you say you will test them and dump them if they are no good. You can also get 12V from old pcs.

[owend]

My unclarity: I should have said that while I was keeping an eye on the LiPos, I was also keeping an eye on the 12v battery, and it hasn't gone over 14.4v.

Good idea on the disabled vehicles' battery: we have hundreds of the electric buggies here, usually backing into me in the supermarket.

Owen

[cjcj1949]

Or you could offer to test them for free. You can get a lot of car batteries too. You can give advice, but many people don't listen, then they give you their useless batteries which are often ok. Older 12V batteries charge very slowly and need a lot of driving to offset the alarm drain. Garages don't test for battery capacity either and a lot of useful batteries get scrapped.

A new battery might need 1 hour of driving a week but an old one might need a lot more.